Using helium, hydrogen, nitrogen, or the like instead of a chlorofluorocarbon as working gas, the Stirling engine has been attracting much attention as a thermal engine that does not destroy the ozone layer. In a Stirling engine for use as a refrigeration machine, a piston is reciprocated in a pressure vessel by a power source such as a linear motor, and, synchronously with the piston, a displacer is reciprocated with a predetermined phase difference kept therebetween. The piston and the displacer allow the working gas to move between a compression space and an expansion space so as to achieve a Stirling cycle (more precisely, in the case of a Stirling refrigeration machine, a reversed Stirling cycle). In the compression space, the temperature of the working gas increases due to isothermal compression; in the expansion space, the temperature of the working gas decreases due to isothermal expansion. In this way, the temperature of the compression space increases and the temperature of the expansion space decreases. Heat dissipation from the compression space (high-temperature space) via a hot heat-conducting head allows the expansion space (low-temperature space) to absorb heat from the outside via a cold heat-conducting head.
As the piston reciprocates continuously, the pressure inside a back pressure space formed around the cylinder that houses the piston gradually increases, and this upsets the pressure balance between the back pressure space and the compression space, causing the center of the reciprocation of the piston to deviate from its original position toward the compression space. This, if not dealt with, may cause the piston to reach its physical movement limit, or may cause the piston and the displacer to collide with each other.
To avoid such a situation, ingenious proposals have been made, as exemplified by the following one: a flow passage is formed in the piston so as to connect the outer circumferential sliding face of the piston to the compression space, a flow passage is formed in the cylinder so as to connect the inner circumferential sliding face of the cylinder to the back pressure space, and when the piston comes to a given position, the two flow passages communicate with each other, thereby keeping the proper pressure balance between the back pressure space and the compression space. An example of such a Stirling engine is disclosed in Patent Publication 1.
In a Stirling engine, the piston is typically driven by a linear motor. The linear motor includes an outer yoke, an inner yoke, and a permanent magnet arranged between them. In a linear motor, a permanent magnet is arranged between an outer yoke and an inner yoke; the magnetic flux density of the magnetic field produced between the outer and inner yokes is thus superposed on the magnetic flux density attributable to the permanent magnet, and the resulting unevenness in magnetic flux density produces a force that makes the piston reciprocate. The piston is coupled to the permanent magnet and thus is allowed to reciprocate. An example of a Stirling engine having such a piston-driving mechanism is disclosed in Patent Publication 2.
Patent Publication 1: JP-A-2002-130853 (pages 3 to 4, FIG. 1, FIG. 11)
Patent Publication 2: JP-A-2003-185284 (pages 2 to 3, FIG. 9)